1. Field of the Disclosure
This disclosure is related to methods for acquiring and processing nuclear magnetic resonance (NMR) measurements for determination of capillary pressure curves in carbonate formations with complex pore systems.
2. Description of the Related Art
More than 50% of the world's hydrocarbon reserves are in carbonate reservoirs. Whereas NMR applications for evaluating siliciclastic sandstone reservoirs are successful, the application of NMR for carbonate rock characterization has always been challenging. Carbonates are characterized by different pore types and complex pore-size distributions. Because of their reactive nature, carbonates undergo a more complicated post-depositional diagenesis compared to siliciclastic sandstones. The diagenesis process includes, among others, cementation, dissolution, dolomitization, recrystallization, and evaporate mineralization. Carbonates are also sensitive to the microorganism activity in their depositional environment. As a result, NMR response to some of these pore typing variations is more complicated rendering prediction of reservoir quality of carbonate rocks a big challenge.
The primary pores in carbonate sediments show a considerably more varied microgeometry than in siliciclastic sediments. Additionally, and in contrast to most siliciclastic rocks, during diagenesis the primary porosity is significantly changed. The predictability of petrophysical properties of carbonate rocks is mainly hampered by this great variability of the connectivity of different pore types. For instance, unconnected, ineffective moldic pores lead to low permeability at a given porosity value, whereas intercrystalline pores in sucrosic dolomite rocks usually show high permeability at the same given porosity. The ability to determine the types of porosity present in a porous rock is vital for reliable prediction of reservoir quality (Lucia, 1995).
Distribution of the pore throats is an important characteristics of a reservoir rock because it determines the magnitude of capillary pressures in a rock and thus (among other things) the saturation-height profile. Formation evaluation methods require measurements of both a pore-throat distribution (capillary pressure curves) and a pore-size distribution (from NMR) of a rock. Among those, at present only NMR measurement can be performed in-situ in a borehole by a logging tool. The rest have to be conducted in laboratory on core samples, and have either a limited range, or are destructive. Developing a reliable method to convert NMR measurements to 1st drainage capillary pressure curves lifts these limitations, and provides a continuous reading of capillary pressure curves with depth.
U.S. Pat. No. 6,977,499 to Kiesl et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, discloses a method of using a nuclear magnetic resonance (NMR) sensor assembly conveyed in a borehole in an earth formation including a carbonate wherein a carbonate classification scheme is used for defining the processing scheme used for processing of the NMR spin echo signals. Parameters of interest such as total porosity, bound volume irreducible (BVI), bound water moveable (BVM), a distribution of transverse relaxation times and/or a distribution of longitudinal relaxation times may be obtained. As in the teachings of Kiesl, samples from the Carbonate NMR Rock Catalog by ART are used, along with other rock databases. It should be noted that while the method of the present disclosure has been illustrated using carbonates as an example, this is not to be construed as a limitation and the method may also be used with siliciclastic rocks.
One embodiment of the disclosure is a method of estimating a value of a property of an earth formation. The method includes: using a nuclear magnetic resonance (NMR) sensor assembly conveyed in a borehole in the earth formation and obtaining nuclear magnetic resonance (NMR) spin-echo signals indicative of the property of the earth formation; processing the spin-echo signals using a radial basis function derived using (i) a database comprising a plurality of samples, and (ii) an NMR measurement associated with each sample in the database and related to the spin echo signals; for estimating the value of the property.
Another embodiment of the disclosure is an apparatus configured to estimate a value of a property of an earth formation. The apparatus includes: a nuclear magnetic resonance (NMR) sensor assembly configured to be conveyed in a borehole in the earth formation and obtain nuclear magnetic resonance (NMR) spin-echo signals indicative of the property of the earth formation; and a processor configured to process the spin-echo signals using a radial basis function derived using (i) a database comprising a plurality of samples, and (ii) an NMR measurement associated with each sample in the database and related to the spin echo signals; for estimating the value of the property.
Another embodiment of the disclosure is a computer-readable medium having thereon instructions that when read by a processor cause the processor to execute a method. The method includes: processing spin-echo signals obtained by a nuclear magnetic resonance (NMR) sensor assembly conveyed in a borehole in the earth using a radial basis function derived using (i) a database comprising a plurality of samples, and (ii) an NMR measurement associated with each sample in the database and related to the Spin echo signals; for estimating the value of the property.